As an in-wheel motor drive device is mounted inside a wheel, and hence increase in weight of the drive device causes increase in unsprung load of a vehicle. The increase in unsprung load causes degradation in traveling stability and NVH characteristics. Consequently, downsizing and lightweighting of the drive device are important. Output torque of an electric motor is proportional to a size and a weight of the motor. Thus, in order to generate torque which is required for drive of the vehicle solely by the motor, a motor having a large size is required. Therefore, in the in-wheel motor drive device, rotation of the electric motor is transmitted to the wheel via a speed reduction mechanism, to thereby achieve the downsizing of the motor.
Incidentally, as disclosed in, for example, Patent Literature 1 (Japanese Patent Application Laid-open No. 2015-214273), an in-wheel motor drive device arranged in a wheel of a vehicle is connected to a suspension device such as a lower arm. For that reason, it is required that an installation space for the suspension device, in addition to the installation space for the in-wheel motor drive device, be secured in the wheel of the vehicle. Moreover, although not shown in the drawings, a brake caliper is also arranged in the wheel, and hence it is also required that an installation space for the brake caliper be secured. Thus, it is desired that a radial dimension of the in-wheel motor drive device be set as small as possible.
Even when the dimension in the radial direction is reduced while sacrificing a dimension in an axial direction (vehicle width direction) to meet such demands, an increase in axial dimension causes increase in protruding amount of the in-wheel motor drive device from the wheel toward an in-board side. As a result, there is a fear in that a vehicle body and the in-wheel motor drive device interfere with each other at the time of steering or vertical movement of the vehicle.
As one example of a structure having reduced in dimensions of the in-wheel motor drive device in the axial direction and the radial direction, for example, in Patent Literature 2 (Japanese Patent Application Laid-open No. 2012-214202), there is disclosed a method of arranging a speed reducer coaxially with a wheel center and arranging the speed reducer and a motor on the same section in the radial direction.